1. Field of the Invention
The present invention relates to a magnetic levitation motor having a stator winding for controllably levitating a rotor and another stator winding for generating a rotation magnetic flux and a method for manufacturing the same.
2. Description of Related Art
Contact-type bearings are widely used. In addition, non-contact type magnetic bearings are also gaining popularity. A typical magnetic bearing uses a magnetic force to levitate a rotor member such as a rotor shaft and supports the rotor member in a non-contact manner. By the use of the magnetic bearing, the coefficient of friction of the bearing section becomes substantially zero (0), which makes a high-speed rotation possible. Also, the magnetic bearing does not need lubrication oil. This allows the use of the magnetic bearing under special conditions. For example, the magnetic bearing can be used at a high temperature or a low temperature, in vacuum and the like. Furthermore, no maintenance work is required. Accordingly, magnetic bearings are used to support rotors in motors.
A motor with magnetic bearing basically has a structure in which a magnetic bearing, a motor section that is a system for generating a rotation force and a magnetic bearing are disposed in this order in an axial direction of a rotor shaft. However, in this structure, the magnetic bearings are disposed on both sides of the motor section, and therefore the length of the rotor shaft increases, and the critical speed lowers.
In view of the fact that the stator of a magnetic bearing has a structure substantially similar to that of a stator of an AC motor, magnetic levitation motors in which magnetic bearings and a motor are formed in one piece have been proposed. One type of magnetic levitation motor is a hybrid magnetic levitation motor. The hybrid magnetic levitation motor uses a permanent magnet to form a constant magnetic flux that radially spreads from within a rotor, so that the rotor is controllably levitated by two-pole direct current magnetic fields, in a similar manner as a typical magnetic bearing. The hybrid magnetic levitation motor can form a constant magnetic flux by the permanent magnet, and therefore can generate a bias attraction force without consuming electric power, which provides an advantage in that an electromagnet used therein can take charge of only the controlling force.
However, in the hybrid magnetic levitation motor described above, relatively large permanent magnets are required to form a constant magnetic flux that radially spreads from within the rotor, in addition to driving permanent magnets for rotating the rotor. This leads to various problems. For example, the number of assembly steps increases, the size reduction of the motors becomes difficult, the manufacturing cost increases and the motors are restricted to a limited number of structures.
Another type of magnetic levitation motor is described in Japanese Laid-open patent application HEI 7-184345. The magnetic levitation motor described in this reference has a magnetic levitation motor section on one side of a shaft member and a magnetic bearing on the other side of the shaft member. However, the magnetic levitation motor described in this reference is difficult to generate a large output appropriate for the overall size of the magnetic levitation motor.
Furthermore, the conventional hybrid magnetic levitation motor is a hybrid of a radial magnetic bearing and a motor. An independent thrust bearing that functions only as a magnetic levitation thrust bearing is added to the hybrid magnetic levitation motor.
On the other hand, compound magnetic bearings that have specially designed magnetic paths are proposed. A typical compound magnetic bearing is formed from a combination of a radial magnetic bearing and a thrust magnetic bearing.
As described above, a conventional magnetic levitation motor is formed from a combination of a radial magnetic bearing and a motor integrally formed in one piece. An independent thrust magnetic bearing that functions only as a thrust bearing is added to such a conventional magnetic levitation motor. The thrust magnetic bearing occupies a substantially large portion of the overall structure of the motor, and therefore is a hindrance to the size reduction of the magnetic levitation motors.
Also, when a compound magnetic bearing formed from a combination of a radial magnetic bearing and a thrust magnetic bearing is employed, an independent motor is additionally required. This also prevents size reduction of the magnetic levitation motors.
It is an object of the present invention to solve the problems of the conventional technology described above. It is also an object of the present invention to provide a magnetic levitation motor having specially designed permanent magnets for generating a bias magnetic flux, which reduces the number of assembly steps, reduces the size of the motor, lowers the manufacturing cost, and improve the degree of freedom in designing the motor structure.
It is also an object of the present invention to provide a magnetic levitation motor that uses a bias magnetic flux of a hybrid magnetic levitation motor, in which a thrust bearing is disposed in a magnetic path of the bias magnetic flux. As a result, a smaller magnetic levitation motor having a compound system of a magnetic levitation motor and a thrust magnetic bearing can be provided.
In accordance with an embodiment of the present invention, a magnetic levitation motor has a rotator body formed from a magnetic member and having segmented permanent magnets attached to a peripheral surface thereof. Two magnetic levitation motor sections are disposed in an axial direction of the rotator body. Each of the magnetic levitation motor sections having a first stator winding that generates a levitation control magnetic flux for levitating the rotator body and a second stator winding that generates a rotation magnetic flux for rotating the rotator body. The segmented permanent magnets are affixed to the rotator body at the two magnetic levitation motor sections with magnetic polarities thereof mutually opposite to each other. The segmented permanent magnets function as bias magnets that generate a direct-current magnetic flux that radially spreads from within the rotor.
As a result, the segmented permanent magnets function as permanent magnets for generating a rotation force and magnets for generating a levitation force. Accordingly, the structure is simplified and reduced in size and the cost is reduced.
In one aspect of the embodiment of the present invention, the two rotors forming the two magnetic levitation motor sections are respectively formed from the segmented permanent magnets. The segmented permanent magnets are mounted on a common rotation member of a magnetic material at different locations in an axial direction of the common rotation member. Furthermore, the two magnetic levitation motor sections includes two stator cores that have an identical structure. A first stator winding and a second stator winding are wound around each of the two stator cores. As a result, the motor output can be increased, and the levitation force that is well balanced in the axial direction is obtained.
In one aspect of the embodiment of the present invention, surfaces of the segmented permanent magnets facing one of the stator cores have an N-pole and surfaces of the segmented permanent magnets facing the other of the stator cores have an S-pole. As a result, the segmented permanent magnets can function as both of permanent magnets for generating a rotation force and permanent magnets for generating a levitation force. As a consequence, the structure is simplified.
In one aspect of the embodiment of the present invention, opposing surfaces of the segmented permanent magnets that face the stator cores have an arc shape to make a gap magnetic flux density to have generally a sine waveform. As a result, mutual interference between the levitation force and the rotation force can be reduced, and the levitation force and the rotation force can be efficiently obtained.
Also, in one aspect of the embodiment of the present invention, the magnetic levitation motor has detecting sections that are integrally mounted on the common rotation member at locations that interpose the two magnetic levitation motor sections, and gap sensors opposing to the detecting sections. A current to the first stator winding for generating a levitation force is controlled such that gaps between the gape sensors and the detection sections detected by the gap sensors are constant. As a result, the common rotation member and the rotating parts including the rotor can be supported in a non-contact manner.
Furthermore, in accordance with another embodiment of the present invention, a magnetic levitation motor has a rotor with a permanent magnet affixed to a peripheral surface thereof, and a stator core section with a first stator winding that generates a levitation control magnetic flux for controllably levitate the rotor and a second stator winding that generates a rotation magnetic field for the rotor. A rotor-side thrust bearing magnetic path section is formed on the rotor, and two stator-side thrust bearing magnetic path sections are formed in a manner to interpose the rotor-side thrust bearing magnetic path section, such that a bias magnetic flux for forming the levitation control magnetic flux passes gaps formed in a thrust direction between the rotor-side thrust bearing magnetic path section and the two stator-side thrust bearing magnetic path sections. A thrust control coil is provided between the two stator-side thrust bearing magnetic path sections. Current is controllably conducted through the thrust control coil to support a thrust bearing load.
With the structure described above, the thrust magnetic bearing can also be incorporated as a compound structure and can be reduced in size. As a result, the length of the shaft can be shortened, and a higher rotation speed can be obtained.
In one aspect of the embodiment of the present invention, the stator core section is formed from two stator core sub-sections arranged in the axial direction, and the rotor-side thrust bearing magnetic path section and the two stator-side thrust bearing magnetic path sections are formed between the two stator core sub-sections.
As a result, in effect, two motor sections are provided in the motor, and thrust loads of the two motor sections are supported by one thrust magnetic bearing. As a consequence, there is provided a relatively compact magnetic levitation motor with a thrust magnetic bearing, which can generate a large output in spite of its compactness.
Also, in one aspect of the embodiment of the present invention, segmented rotor magnets for generating a rotation torque are disposed on the rotor opposite to the stator core section, wherein the segmented rotor magnets also function as bias magnets for generating a bias magnetic flux.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.